Since multilayer headboxes were first proposed, it has been the conviction of those versed in the art that the velocities of the adjacent discharged jets must be as nearly the same as possible. To this end, headboxes have been devised in which the several stock flow channels are formed by self-positionable partitions fixed at the upstream end and free at the downstream end, as disclosed in the prior U.S. Pat. Nos. 3,923,593 to Verseput and 4,141,788 to Justus. The maintenance of a constant thickness for the individual layers in the web has been a continuing problem with such headboxes, and efforts to accomplish this result by adjusting the pressures in the stocks flowing in the several channels have not been effective.
An attempt has been made to solve this problem by using as a channel separator a thin, resilient plastic foil fixed in the headbox at its upstream end and fastened elastically at points along each of its two side edges to e.g. the adjacent sidewalls of the headbox, as shown in U.S. Pat. No. 4,243,483 to Schiel et al. This design contemplates discharge of the individual layers of the stock jet at equal velocities relative to each other. It does not permit the layers to be discharged at different relative velocities as required for the production of multilayer paper of superior quality in certain applications.
It has also been proposed, as disclosed in U.S. Pat. No. 4,181,568 to V. Pfaler, to form the channels in a multilayer headbox by relatively rigid separator vanes pivotally mounted at the upstream ends and with their downstream ends free, and to provide means for displacing the upstream ends of the vanes in a direction perpendicular to the plane of the vane to enable adjustment of the ratios of the cross-sectional areas of the stock channels to make them the same as the ratios of the stock volumes flowing through the stock channels per unit time. In this way, pressure difference that would tend to displace a separator vane from the position where the discharged layers of the stock jet have the same relative velocity cannot arise. Therefore, this headbox is not capable of operating to discharge a jet in which adjacent layers have different relative velocities.
Multilayer headboxes have also been devised in which the flow channels are formed by relatively rigid partitions firmly clamped to the headbox sidewalls, with provision for limited positional adjustment, as shown in prior U.S. Pat. No. 4,372,816 to Wolf et al. In a headbox of this type, having slice channels with linear convergence, if the ratio between the cross-sectional areas of the individual slice channels at the upstream ends of the partitions is fixed and unchangeable, and is the same as the ratio between the stock flows, there is only one position for each partition in which the partition will not be subjected to any bending moment, its points of attachment to the sidewalls will not be subjected to any shearing stresses, and the layers in the stock jet will be discharged at the same velocity relative to each other.
For every other position, there is only one stock flow ratio at which the points of attachment of the partition to the sidewalls will not be subjected to any shearing stresses, but at which the partition experiences a bending moment in the longitudinal direction and the layers in the stock jet will be discharged at different velocities relative to each other. At all other combinations of stock flow ratios and partition positions, the points of attachment of the partitions to the sidewalls will be subjected to shearing stresses which might damage the mounting, and the partition will also experience a bending moment in the transverse direction, so that even when constructed as rigidly as possible, the run of it between the sidewalls will not be straight but curved. As a result, the thickness of the discharged layers of stock will vary along the width of the stock jet.
It will be readily appreciated, therefore, that, in wide papermaking machines particularly, it is very difficult to adjust the settings of the partitions in relation to the ratios between the stock flows in the slice channels in such a way that the thicknesses of the individual stock layers of the stock jet discharged from a multilayer headbox of this kind can be maintained constant along the width of the stock jet. Further, if the ratio between the stock flows is not in agreement with the ratio between the cross-sectional areas of the slice channels, the individual layers will also be discharged at different velocities, which is considered by some of those versed in the art to be detrimental to the production of high grade multi-ply paper.